Synthesis, spectral characterization and crystal structures of organophosphonic diamides: pyramidal nitrogen centers and hydrogen bonding in [PhP(O)(NHtBu)2], [PhP(O)(NHDipp)2] (Dipp = 2,6-iPr2C6H3) and [tBuP(O)(NHiPr)2

Abstract

Organophosphonic diamides of general formula [R1P(O)(NHR2)2] (1–8; R1 = Ph, Cy or tBu and R2 = Cy, iPr, tBu or Dipp) have been prepared by the addition of P,P'-dichloro(alkyl/aryl)phosphine oxide, R1P(O)Cl2, to a solution of primary amine in toluene. The synthesis of PhP(O)(NHtBu)2 (1) in high yield was achieved by slow oxidation of PhP(NHtBu)2 in air over several days. All new compounds have been characterized by elemental analysis and by IR, EI mass and NMR (1H and 31P) spectroscopy. The molecular structures of [PhP(O)(NHtBu)2] (1), [PhPO(NHDipp)2] (2) and [tBuP(O)(NHiPr)2] (7) have been determined by single crystal X-ray diffraction studies. The amido nitrogen atoms in 1, 2 and 7 show considerable deviation from the expected trigonal-planar geometry. The observed dihedral angles and the orientation of the nitrogen lone pair (l.p.) in these compounds point to the role of l.p.(N) → σ*(P–X) type negative hyperconjugative interactions in P–N multiple bonding. Compounds 1 and 7 form interesting polymeric structures in the solid state with the aid of N–H⋯OP hydrogen bonding interactions while the presence of the very bulky Dipp substituent on nitrogen in 2 prevents the participation of the N–H protons in hydrogen bonding with phosphoryl oxygen atoms.